1. Field of the Invention
This invention relates to steam turbines and, more particularly, to a method and apparatus for utilizing spillover steam from the high pressure steam turbine gland casings as a source of sealing steam for a low pressure turbines and other steam turbines employed in the steam power plant.
2. Description of the Related Art
It has been common to utilize a combination of high and low pressure steam turbines in nuclear plants or other steam powered industrial facilities to produce electricity. Typically, these turbines were used to extract enthalpy and kinetic energy from heated process steam produced by some source, such as a nuclear reactor or nuclear/fossil fuel heated boiler. The enthalpy and kinetic energy in the process steam was converted into kinetic energy, which was used to drive a shaft partially enclosed within the turbine casings and coupled to an electrical generator. At the points where the shaft penetrated the turbine casings, process steam would occasionally leak out from within the turbine casings and into the atmosphere, or air from the atmosphere would sometimes leak into the turbine casings. The steam leakage out of the casings would result in diminished turbine output and water contamination of the lubricating oil system causing degraded lubrication capability and corrosion to the lube oil system and the components it serves. The air leakage into the casings would reduce the vacuum within the main condensers and hamper steam plant efficiency.
One technique that has been utilized to prevent such leakage is to supply sealing steam to the turbine glands. The sealing steam was used, along with other components, to create a leak-proof barrier between atmospheric air and the process steam in the turbine chest. High pressure steam turbines required this sealing steam only during turbine startup or at lower generator outputs. Under normal operating conditions of high generator output, the high pressure turbines and one directional low pressure turbines are generally “self-sealing.” Specifically, exhaust steam was emitted from the turbine casing into the gland casing with sufficient pressure to serve as a source of sealing steam in the turbine glands. As a result, the high pressure turbine did not require sealing steam from an external source. In contrast, two direction low pressure steam turbines required a external source of sealing steam at all times during operation, regardless of the level of generator output.
It has often proven difficult to provide a convenient, efficient and cost-effective source for low pressure turbine gland sealing steam. Various techniques have been proposed in the prior art for providing this necessary sealing steam. For example, U.S. Pat. No. 3,935,710 to Dickenson shows a portion of the process steam being heated and used as low pressure turbine gland sealing steam. In this regard, reduction of steam pressure from line to near-atmospheric level typically provides sufficient superheating for use on two-direction low pressure turbines. In many cases, the pressure reduction alone produces sufficient superheating. So far as is known, however, the technique in Dickenson was not adapted or suitable for use in treating potential sources of sealing steam other than process steam from the steam plant, such as turbine exhaust steam. Alternatively, U.S. Pat. No. 4,541,247 to Martin shows exhaust steam being extracted from the high pressure turbine casings and then desuperheated and used as low pressure turbine gland sealing steam. Because of the desuperheated steam, the techniques of this patent are not adapted to a number of situations, such as for example with waterlogged high pressure exhaust steam, where steam requires superheating.